The complex interrelationships between cell surface membranes and differentiation will be investigated using differentiating skeletal muscle as a model system. Skeletal muscle differentiation in vitro, as well as in vivo, proceeds through several stages culminating in the fusion of mononucleated myogenic cells into multinucleated myotubes which develop striations and become contractile. Since the cell surface or plasma membrane must play a key role in muscle differentiation, viz. cell recognition and cell fusion, we will examine in detail the surface membranes of cultured chick embryo muscle cells as a function of stage of differentiation. Various culture techniques will be utilized to synchronize the differentiation process and enrich cultures for cells at specific stages of differentiation. At each of these stages, plasma membranes will be isolated and the composition, metabolic turnover, and spatial distribution or topography of the membrane components will be compared. It is anticipated that these studies will: 1) provide a better understanding of the role of the plasma membrane in cellular differentiation, in general, and in striated muscle differentiation, in particular; and 2) provide new information on the mechanism of naturally occurring membrane fusion.